Gabor invented holography in 1948. It involves irradiating an object with a radiation beam of strong coherence. The waves which are scattered by the object interfere with the background wave on a photographic film or digital image sensor, where interference patterns are recorded. Based on the recorded interference pattern it is possible to reconstruct the original object wave field.
Since digital image sensors (camera sensors organized into semiconductor matrix based for instance on CCD, CMOS and similar configurations), digital holography has gained grounds. Holograms exposed on conventional photo materials need to be treated physically or chemically. This need is removed in the field of digital holograms. Moreover, the recorded hologram information can be stored in digital memory. The stored hologram can also be improved by appropriate software algorithms on the basis of the digital information only. The spatial image can for instance also be reconstructed from the hologram in a numerical way; there is no need for an illuminating light source and other optical means for the reconstruction of the spatial image.
Lens-free imaging is a microscopic imaging method based on the principal of in-line holography. In-line holography is a commonly used holographic imaging technique, and is known because of its simplicity and minimal optical hardware requirements. The in-line holography technique however suffers from the so-called twin-image problem, being the inherent artifact of out-of-focus virtual object information appearing in the in-focus real object image. Some techniques can eliminate the twin-image artifact. They can be categorized in two groups. A first group is constituted by support-/mask-based methods, which cannot be applied in complex object imaging as it is impossible to create a good mask for an object without obscuring the object right next to it. The other group is constituted of mask less iterative phase retrieval methods, which rely on acquisition of multiple images with varying information content. These multiple images with varying information content can be acquired by varying the phase, the imaging distance or the illumination wavelength between the different subsequent acquisitions. The multi-image acquisition can either performed in the time domain, for instance sequentially recording image after image, or in the space domain, for instance splitting up the light into multiple optical parts to vary the phase of light arriving at the sensor (at the pixel level) or to vary the optical path (at sensor level).
The multi-image acquisition performed in the time domain is not suitable for high speed imaging (e.g. in real-time video applications) as the acquisitions at different time-instances will typically record slightly or seriously different object perspectives (for instance objects rotating or shifting). The multi-image acquisition performed in the spatial domain, realized in the prior art for example using a beam splitter, has limited performance due to the fact that the imaging conditions cannot be optimized due to the presence of the extra optical hardware. For instance, a beam splitter does itself restrict the minimal distance from object to imager. This results in serious resolution degradation of the final reconstructed image.
Today, there exists a need for apparatuses and methods which are suitable for performing high-speed high resolution in-line lens-free digital imaging.
In WO2012/150472 an apparatus for producing three-dimensional color images is disclosed, the apparatus comprising at least two feeding light sources generating coherent light beams of different colors, at least two optical fibers having input ends and light emitting ends, the input ends of the optical fibers being connected to the feeding light sources, respectively, the light emitting ends of the optical fibers being placed closely side by side and constituting an illuminating light source, an object space suitable for locating an object to be illuminated by the illuminating light source, at least one digital image sensing device for recording an interference pattern of reference light beams and object light beams scattered on or reflected by the object as a hologram, and the digital image processing device for producing the three dimensional color images of the object from the hologram recorded by the at least one digital image sensing device with a correction of distortions resulting from placing side by side the light emitting ends of the optical fibers.
Here, a plurality of feeding light sources each emit light with a very narrow wavelength spectrum, from neighboring but still substantially different locations, such that a correction of distortions is necessary. Such a configuration puts stringent conditions on the light sources used, making it an expensive solution. Moreover, multiple sources are needed, which again increases the cost and results in relatively large devices. Also, at the digital image sensing device, wavelength filters with a relatively broad wavelength range are applied, in order to be able to extract information for different colors.